Integrated polarization coupler

ABSTRACT

A polarization coupler includes a stress-inducing feature disposed to generate a stress-field in a substrate. First and second waveguides each have a coupling portion that pass through the stress-field. First and second periodic-structures are in optical communication with the coupling portions of the first and second waveguides.

BACKGROUND

The invention relates to integrated optical devices, and in particular,to polarization couplers.

A known method for amplifying an optical signal beam propagating in awaveguide is to pass a pump beam through the same waveguide. In thismethod, known in the art as “Raman amplification,” the pump beamtransfers energy to phonons within the waveguide. If the wavelength ofthe pump beam is correctly chosen, the energy in these phonons istransferred to the signal beam, thus amplifying the optical signal.

In most cases, the optical signal occupies a band of wavelengths. Asingle pump beam can only amplify a limited portion of the entire bandoccupied by the optical signal. As a result, it is often necessary tocombine several pump beams, each at a slightly different wavelength, toprovide amplification over the entire band occupied by the opticalsignal.

The extent to which a pump beam amplifies the signal beam also dependson the polarization difference between the pump beam and the signalbeam. The pump beam, having been generated by a nearby laser, istypically linearly polarized. The signal beam, having been generated faraway, has become thoroughly depolarized. As a result, the amplificationof the signal beam will depend on whether the linearly-polarized pumpbeam and the randomly polarized signal beam happen to share the samepolarization.

SUMMARY

In one aspect, the invention includes a polarization coupler in which astress-inducing feature is disposed to generate a stress-field in asubstrate. First and second waveguides, each having a coupling portion,pass through the stress-field. First and second periodic-structures arein optical communication with the coupling portions of the first andsecond waveguides.

In one embodiment, the first periodic-structure includes a grating. Thisgrating can be a short-period grating or a long-period grating.

The first and second periodic-structures are either aligned with eachother or are offset from one another. The offset between the first andsecond periodic-structures can be along the axial dimension of thewaveguide or a transverse dimension.

One example of a stress-inducing feature is a stress-inducing stripdisposed on the surface of the substrate. Such a stress-inducing stripcan be placed above the first waveguide or it can extend across thefirst and second waveguides. One way to generate a stress-field is toselect a strip material having a coefficient of thermal expansion thatdiffers from a coefficient of thermal expansion of the substrate.

The first and second waveguides have first and second cross-sectionsrespectively. In some embodiments, the first and second cross-sectionsare the same. However, the invention also includes those embodiments inwhich the first and second cross-sections are different from each other.

In another aspect, the invention includes an integrated optical circuiton a substrate having a birefringent portion. First and secondwaveguides having respective first-waveguide and second-waveguidecoupling sections pass through the birefringent portion. Afirst-waveguide periodic-structure is in optical communication with thefirst-waveguide coupling-portion and a second-waveguideperiodic-structure is in optical communication with the second-waveguidecoupling-portion.

Another aspect of the invention is a method for combining a first lightwave having a first polarization and a second light wave having a secondpolarization. The method includes inducing a stress field in a substrateand guiding the first and second light waves through the stress field inrespective first and second waveguides. The first and second waveguideseach have a coupling portion in optical communication with acorresponding periodic-structure.

The polarization coupler can be integrated into a substrate and cantherefore be easily manufactured using conventional process steps.

These and other features and advantages of the invention will beapparent from the following detailed description and the accompanyingfigures, in which:

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1-4 show embodiments of a wavelength polarization multiplexer.

FIGS. 5-6 show a polarization rotator used in one of the wavelengthpolarization multiplexers of FIGS. 1-4.

FIGS. 7-9 show plan, perspective and cross-sectional views respectivelyof a polarization coupler that can be used in any of the wavelengthpolarization multiplexers of FIGS. 1-4.

FIGS. 10-11 show alternative arrangements for a polarization couplerstructure.

FIGS. 12-13 show alternative stress-inducing structures.

DETAILED DESCRIPTION

FIG. 1 shows a first embodiment 8A of an optical multiplexer thatcombines several beams having different wavelengths and polarizationsinto one output beam. The first embodiment 8A includes a planarsubstrate 10 having an input 12 adjoining an input coupling region 14and an output 16 adjoining an output coupling region 18. A plurality oflaser diode pairs 20A-B, 20C-D, 20E-F, 20G-H couple their light energyto corresponding inputs in the input coupling region 14. For instance,first and second laser diodes 20A-B, which emit light at the samewavelength with the same polarization (herein referred to as the“principal polarization”), pass first and second beams into the inputcoupling region 14.

Each laser diode pair 20A-B emits light at a wavelength that differsfrom the wavelengths emitted by other laser diode pairs 20C-D, 20E-F,20G-H. The number of laser diode pairs, and hence the number ofwavelengths propagating within the optical multiplexer, can be varied tosuit the application of the optical multiplexer.

The first beam is coupled into a first waveguide 22A that extends fromthe input 12 to the output face 16. The second beam is coupled into asecond waveguide 22B that extends from the input 12 to a firstpolarization coupler 24A proximate to the first waveguide 22A. The firstand second beams are coupled to the respective first and secondwaveguides 22A, 22B by a lens coupling system (not shown).

The currents driving each laser diode 20A-H are controlled by anexternal control circuit (not shown). The control circuit providesselected currents to the respective laser diodes 20A-H to achieve gainflattening across the laser diodes 20A-H. In doing so, the controlcircuit relies, to some extent, on a feedback signal indicative of thepower output of a particular laser diode. For a particular laser diode20G, a feedback signal can be obtained by providing a tap 25 thatextracts, from a waveguide 22G, a portion of the energy delivered tothat waveguide 22G by its associated laser diode 20G.

Between the input 12 and the first polarization coupler 24A, the secondwaveguide 22B passes through a first polarization rotator 26A that isintegral with the substrate 10. As used herein, “polarization rotator”refers to a two-port device that rotates the polarization of lightpassing therethrough by a selected angle. Typically, this angle is anodd multiple of ninety degrees, in which case the polarization rotator26A transforms a principally polarized beam at its input into anorthogonally polarized beam at its output.

The first and second laser diodes 20A-B are identical in structure andconfiguration. As a result, the first and second beams are bothprincipally polarized. After passing through the first polarizationrotator 26A, the second beam, which is now orthogonally polarizedrelative to the first beam, proceeds to the first polarization coupler24A. At the first polarization coupler 24A, the orthogonally polarizedsecond beam is added to the principally polarized first beam alreadypropagating on the first waveguide 22A.

As used herein, “polarization coupler” refers to a four-port deviceintegrated into the substrate 10 that combines orthogonally polarizedbeams presented at its first and second input ports and provides theresulting combined beam at a first output port. A second output port ofthe polarization coupler terminates in the substrate 10.

Third and fourth beams from respective third and fourth laser diodes20C-D, both of which emit light at a second wavelength, are coupled tothird and fourth waveguides 22C-D in the same manner described above inconnection with the first and second laser diodes 20A-B. The thirdwaveguide 22C extends from the input 12 to a first wavelength coupler28A proximate to the first waveguide 22A.

As used herein, “wavelength coupler” refers to a four-port deviceintegrated into the substrate 10 that combines beams of differentwavelengths present at its first and second input ports and provides theresulting combined beam at a first output port. A second output port ofthe wavelength coupler terminates in the substrate 10. The use ofwavelength specific coupling devices, such as a wavelength coupler,avoids the possibility that beams of other wavelengths that are alreadypropagating on the first waveguide 22A will leak out through a couplereach time an additional beam is placed on the first waveguide 22A.

At the first wavelength coupler 24A, the third beam is combined with thefirst and second beams already propagating on the first waveguide 22A.The fourth waveguide 22D extends from the input 12, through a secondpolarization rotator 26B, to a second polarization coupler 24B, asdescribed above in connection with the second waveguide 22B. As aresult, the fourth beam, which has a polarization orthogonal to thethird beam, is also combined with the beams already propagating on thefirst waveguide 22A. Thus, at the output of the second polarizationcoupler 24B, the first waveguide 22A now carries orthogonally polarizedbeams at the first wavelength and orthogonally polarized beams at thesecond wavelength.

The foregoing pattern continues, with fifth and sixth laser diodes 20E-Femitting fifth and sixth beams at a third wavelength and seventh andeighth laser diodes 20G-H emitting seventh and eighth beams at a fourthwavelength. The fifth and seventh beams are guided to second and thirdwavelength couplers 28B-C by fifth and seventh waveguides 22E, 22G. Thefifth and seventh beams are thus added, with their principalpolarizations intact, to the beams already propagating in the firstwaveguide 22A.

Meanwhile, the sixth and eight beams emitted by sixth and eighth laserdiodes 20F, 20H are coupled to sixth and eight waveguides 22F, 22H. Thesixth and eighth waveguides 22F, 22H guide the sixth and eight beamsthrough third and fourth polarization rotators 26C-D and on to third andfourth polarization couplers 24C-D. The third and fourth polarizationcouplers 24C-D add the sixth and eighth beams, which have now had theirpolarizations rotated, to the beams that are already propagating on thefirst waveguide 22A.

The first waveguide 22A extends to the output face 16 of the substrate10, carrying with it all eight constituent components collected as ittraversed the substrate 10 from the input 12 to the output 14. This beamis coupled, across the coupling region 18, into an output waveguide 30.

There are additional ways to arrange polarization rotators, wavelengthcouplers, and polarization couplers on the substrate 10 to achieve thesame result as that illustrated in FIG. 1.

FIG. 2 shows a second embodiment 8B embodiment for an opticalmultiplexer. In this second embodiment 8B, the second waveguide 22Bcollects all four orthogonally polarized beams, which are at fourdifferent wavelengths and directs them to a broadband polarizationcoupler 32. The broadband polarization coupler 32 combines these fourorthogonally polarized beams with the four principally polarized beamsalready propagating on the first waveguide 22A.

In the second embodiment 8B, the first waveguide 22A passes throughfirst, second, and third wavelength couplers 28A-C. These wavelengthcouplers 28A-C add the third, fifth, and seventh beams, all of which areprincipally polarized, to the first beam (also principally polarized)already propagating on the first waveguide 22A. Similarly, the secondwaveguide 22B passes through three different wavelength couplers 28D-F.At each of the wavelength couplers the second waveguide 22B collects abeam of a different wavelength whose polarization has been rotated by apolarization rotator 26A-D. The second waveguide 22B carries itscollected orthogonally polarized beams into the broadband polarizationcoupler 32. The broadband polarization coupler 32 couples theorthogonally polarized light to the first waveguide 22A. The secondembodiment 8B shown in FIG. 2 thus requires only one, albeit broadband,polarization coupler 32 rather than the four narrowband polarizationcouplers 24A-D shown in FIG. 1.

FIG. 3 shows a third embodiment 8C similar to the second embodiment 8Bshown in FIG. 2, the principal difference being that the beams collectedby the second waveguide 22B at each wavelength coupler 28D-F retaintheir principal polarization. The second waveguide 22B brings thesecollected beams, all of which are at different wavelengths, to abroadband polarization rotator 34. At the broadband polarization rotator34, the polarizations of the collected beams are all rotated together.The second waveguide 22B directs the now orthogonally polarized beams tothe broadband polarization coupler 32, which places then all at onceinto the first waveguide 22A. Like the embodiment of FIG. 2, theembodiment shown in FIG. 3 requires only a single polarization coupler32. However, it also requires only a single, albeit broadband,polarization rotator 34 instead of the four narrowband polarizationrotators 26A-D shown in FIGS. 1 and 2.

FIG. 4 shows a fourth embodiment 8D that, like the first embodiment 8A,relies on four narrowband polarization rotators and four narrowbandpolarization couplers. In this fourth embodiment 8D, the principallypolarized beams and the orthogonally polarized beams at each wavelengthare combined before being placed on a first waveguide 22A.

In FIG. 4, first and second beams from the first and second laser diodes20A-B are coupled across the coupling region 14 to the first and secondwaveguides 22A-B. The second waveguide 22B directs the second beamthrough a first polarization rotator 26A and on to a first polarizationcoupler 24A. The second beam, which is now orthogonally polarized, isadded to the first beam already propagating on the first waveguide 22A.

Principally polarized beams of the same wavelength, provided by thirdand fourth laser diodes 20C-D, are coupled to third and fourthwaveguides 22C-D in the same way. The fourth waveguide 22D directs thefourth beam to a second polarization coupler 24B. The third waveguide22C directs the third beam through a second polarization rotator 26B,which rotates it into a orthogonally polarized beam and guides it to thesecond polarization coupler 24B. The second polarization coupler 24Badds the fourth beam to the now orthogonally polarized third beamalready propagating on the third waveguide 22C. The combination of theprincipally polarized beams and the orthogonally polarized beams, bothof which have the same wavelength, is then added to the first waveguide22A by a first narrow-band wavelength coupler 28A.

The foregoing procedure is repeated for additional pairs of laser diodes20E-F, 20G-H. One 20E, 20H of each pair of laser diodes is coupled, by awaveguide 22E, 22H, to a polarization rotator 26C-D and the other 20F-Gis coupled, by another waveguide 22F-G, to a polarization coupler 24B-C.The polarization rotator 26C-D is coupled to the polarization coupler24B-C by the waveguide 22H, 22E.

FIGS. 5 and 6 show an exemplary polarization rotator 26A that changesthe polarization state of a beam by passing that beam through abirefringent portion 27A of a waveguide 22A. The birefringent portion27A has a principal axis that is rotated relative to the polarizationvector of light provided by the laser 20A feeding that waveguide 22A. Inthe illustrated polarization rotator 26A, rotation of the principal axisis induced by creating a local stress field in the birefringent portion27A or by locally perturbing a uniform stress field in the birefringentportion 27A. In either case, the direction of the local stress field isoffset from the direction of the polarization vector, thereby causingthe principal axis within the birefringent portion 27A to be neitherparallel nor perpendicular to the polarization vector.

In operation, linearly polarized light from a laser 20A propagates onthe waveguide 22A toward the birefringent portion 27A thereof. Once thislight reaches the birefringent portion 27A, the polarization vectorresolves into a first component that is parallel to the principal axisand a second component that is orthogonal to the principal axis. Thefirst and second components then propagate at different velocities. Asthey do so, the polarization vector begins to rotate. Once the lightleaves the birefringent portion 27A of the waveguide 22A, these twocomponents propagate at the same velocity, thereby freezing the rotationof the polarization vector. By properly selecting the length of thebirefringent portion 27A and the angle of the principal axis relative tothe polarization vector of incoming light, one can freeze theorientation of the polarization vector at any desired angle.

Various structures can be used to cause the desired stress field in thebirefringent portion 27A of the waveguide 22A. One such structure is astress-applying strip 38 on the surface 40 of the substrate 10. Thestress-applying strip 38 is made of a material having a coefficient ofthermal expansion that is different from that of the underlyingsubstrate 10. For example, if the substrate 10 is glass, the strip 38can be silicon or metal. Suitable materials for use in a strip includematerials having a high coefficient of thermal expansion, such asmetals, glass compositions having a high coefficient of thermalexpansions, such as boron doped silica, and polymers having a highcoefficient of thermal expansion.

The strip 38 is deposited onto the surface 40 in a high-temperatureprocess, during which both the substrate 10 and the strip 38 are in anexpanded state. When the substrate 10 and strip 38 cool, they contractby different amounts. Because the strip 38 is physically attached to thesubstrate 10, this results in a stress field near the strip 38. Thisstress field changes the optical properties of structures in regions ofthe substrate 10 near the strip 38.

Stress applied to the substrate 10 causes a shifting of the atomicpositions and electron cloud distributions within the substrate 10. Anelectromagnetic wave sees these two effects cumulatively as a change inthe index of refraction. Because the stress is not equal in alldirections, waves having different polarizations experience differentindices of refraction. The distribution of the induced stress in thesubstrate 10 is calculated by finite element modeling using knownconstitutive relations between stress and the rotation of the principalaxis in response to that stress. The extent of this rotation depends onthe change in the index of refraction in each direction. A relationshipbetween the induced stress and the change in the index of refraction isgiven by: $\begin{Bmatrix}{\Delta\quad n_{xx}} \\{\Delta\quad n_{yy}} \\{\Delta\quad n_{zz}} \\{\Delta\quad n_{xy}} \\{\Delta\quad n_{xz}} \\{\Delta\quad n_{yz}}\end{Bmatrix} = {{- \begin{bmatrix}B_{11} & B_{12} & B_{12} & 0 & 0 & 0 \\B_{12} & B_{11} & B_{12} & 0 & 0 & 0 \\B_{12} & B_{12} & B_{11} & 0 & 0 & 0 \\0 & 0 & 0 & B_{44} & 0 & 0 \\0 & 0 & 0 & 0 & B_{44} & 0 \\0 & 0 & 0 & 0 & 0 & B_{44}\end{bmatrix}} \cdot \begin{Bmatrix}\sigma_{xx} \\\sigma_{yy} \\\sigma_{zz} \\\tau_{xy} \\\tau_{xz} \\\tau_{yz}\end{Bmatrix}}$where Δn_(ij) is the change in the index of refraction from itsstress-free value in the Cartesian directions i and j, B is thestress-optic tensor, which is a measured material constant, and σ_(ij)and τ_(ij) are the stresses and torsions in the Cartesian directions.

As shown in FIG. 5, the strip 38 is deposited proximate to, but notdirectly above, the waveguide 22A, with the longitudinal direction ofthe strip 38 being parallel to the longitudinal axis of the waveguide22A. The waveguide 22A passes through the stress-field generated by thestrip 38. The direction of the stress-field through which the waveguide22A passes is selected to cause a portion of the waveguide 22A to becomea birefringent portion 27A having a principal axis that is offset fromthe polarization vector of light that is to feed that waveguide 22A. Theextent of this stress-induced birefringence, and the direction of theresulting principal axis, depends on the differences between thecoefficients of thermal expansion of the substrate 10 and the strip 38,as well as on the position of the waveguide 22A relative to the strip38.

The length of the strip 38 is selected to rotate the polarization of abeam propagating on the waveguide 22A by the desired angle. Thisdimension thus depends on the extent of the stress-induced birefringencewithin the waveguide 22A. For example, if the birefringence is such thatthe principal and orthogonal axes within the birefringent portion 27A ofthe waveguide 22A are rotated by 45 degrees relative to the polarizationvector, then a strip 38 that is approximately a quarter-wavelength longwill provide a ninety degree rotation of the beam's polarization. Oneformula that relates the length of the strip to the extent of thebirefringence is: $\frac{\lambda}{2\quad L} = {\delta\quad n}$where λ is the wavelength of interest, L is the length of the strip, andδn is the difference between the indices of refraction of the principaland orthogonal axes. For a given geometry, the indices of refraction canbe obtained by finite-element modeling to obtain the stress distributionwithin the substrate 10, and by application of the stress-optic tensorto relate the stress distribution thus calculated to the opticalproperties of the substrate 10.

An exemplary polarization coupler 24A, shown in FIGS. 7-9, couples anorthogonally polarized beam propagating in an incoming waveguide (whichin this case is the second waveguide 22B) to a principally polarizedbeam already propagating in an identical through waveguide (which inthis case is the first waveguide 22A). The polarization coupler 24Acouples the orthogonally polarized beam while preventing the principallypolarized beam in the first waveguide 22A from being coupled into thesecond waveguide 22B. Moreover, any principally polarized componentpropagating on the second waveguide 22B is excluded from the firstwaveguide 22A.

As shown in FIG. 7, a bend 42 in the second waveguide 22B brings acoupling portion 44B thereof into proximity with a coupling portion 44Aof the first waveguide 22A. First and second gratings 46A-B are disposedwithin or above the coupling portions 44A-B of the first and secondwaveguides 22A-B respectively. Depending on the direction in which thecoupled wave from the second waveguide 22B is intended to propagate, thegratings 46A-B can be long-period gratings or short-period gratings. Inaddition, any periodic structure suitable for coupling waves from onewaveguide to another can be used in place of the gratings 46A-B. Thestructure and operation of gratings for coupling light betweenwaveguides is fully discussed in “Fiber Grating Spectra” by TuranErdogan, Journal of Lightwave Technology, Vol 15, No 8, August 1997, thecontents of which are herein incorporated by reference.

In the embodiment shown in FIGS. 7-9 the waveguide structures areidentical in cross-sectional size and construction so as to providecoupling in a forward propagating mode. The second grating 44B couples aforward propagating mode in the second waveguide 22B into a forwardpropagating mode within the substrate 10. The first grating 46A couplesthe forward propagating mode within the substrate 10 into the firstwaveguide 22A.

FIG. 10 shows an alternative embodiment, of a polarization coupler inwhich the first and second waveguides 22A-B have slightly differentdimensions. In this case, an evanescent mode, rather than a forwardpropagating mode, is present in the substrate 10. An embodiment thatrelies on evanescent mode coupling across the gap between the twowaveguides 22A-B can provide more efficient coupling than one thatrelies on the forward propagating mode because the waveguides 22A-B canbe brought closer together without introducing significant broadbandcoupling between the two waveguide 22A-B.

Light is coupled from the second waveguide 22B into the first waveguide22A only when the propagation constants on the first and secondwaveguides 22A-B match. Hence, to couple only the orthogonalpolarization, the propagation constants for the principal polarizationon the first and second waveguides 22A-B must be different.

As shown in FIGS. 8-9, a stress-applying strip 48 is placed on thesurface 40 of the substrate 10 directly above the coupling region 44B ofthe second waveguide 22B shown in FIG. 7. The stress-applying strip 48is made of a material having a coefficient of thermal expansion that isdifferent from that of the underlying substrate 10. For example, if thesubstrate 10 is glass, the strip 48 can be silicon or metal. Thematerial can be one having a high coefficient of thermal expansion, suchas a metal, a glass composition having a high coefficient of thermalexpansion, such as boron doped silica, or a polymer having a highcoefficient of thermal expansion.

The strip 48 is deposited onto the surface 40 in a high-temperatureprocess during which both the substrate 10 and the strip 48 are in anexpanded state. When the substrate 10 and strip 48 cool, they contractby different amounts. Because the strip 48 is physically bonded to thesubstrate 10, these differences in coefficient of thermal expansioncause forces that result in a stress field in a neighborhood of thestrip 48.

As shown in FIG. 9, the strip 48 is deposited proximate to, but notdirectly above, the first waveguide 22A, with the longitudinal directionof the strip 48 being parallel to the longitudinal axis of the firstwaveguide 22A. The first waveguide 22A is thus subjected to asymmetrictransverse stresses. These asymmetric transverse stresses cause thematerial within the first waveguide 22A to become birefringent. Theextent of this birefringence depends on the differences between thecoefficients of thermal expansion of the substrate 10 and the strip 48,the position of the first waveguide 22A and the second waveguide 22Brelative to the strip 48, and the positions of the first and secondgratings 46A-B relative to the first and second waveguides 22A-B.

The second waveguide 22B, which is directly under the strip 48, issubjected only to symmetric transverse stresses. As a result, nobirefringence is induced within the second waveguide 22B. The resultingdifference between the propagation constants for the principalpolarization in the second waveguide 22B and the first waveguide 22Aprevents principally polarized modes from coupling from the firstwaveguide 22A into the second waveguide 22B. To the extent thatpropagation constants for the orthogonal mode remains the same, theorthogonally polarized mode is coupled from the second waveguide 22Binto the first waveguide 22A.

As shown in FIG. 9, the strip 48 is directly above the second waveguide22B. However, there can also be embodiments in which the strip 48 isdisposed directly above the first waveguide 22A.

In addition, the embodiment shown in FIG. 9 has two gratings 46A-B, oneabove each of the waveguides 22A-B. However, a polarization coupler canalso be made by providing a single grating that extends across bothwaveguides 22A-B, as illustrated in FIG. 11.

The structure described herein can also be used in reverse. In such anapplication, a principally polarized component and an orthogonallypolarized mode propagate on the first waveguide 22A. Upon reaching thecoupling region, the orthogonally polarized component is coupled intothe second waveguide 22B while the principally polarized componentcontinues through the first waveguide 22A.

In some embodiments of either the polarization rotator 26A or thepolarization coupler 24A, the strip 38 can be a highly resistivematerial that expands in response to ohmic heating by an electricalcurrent. In other embodiments, the strip 38 can be a piezo-electricmaterial, in which case the strip 38 can be deformed in response to anapplied voltage. Both these embodiments allow fine-tuning of thebirefringence characteristic of the structure, either by varying thecurrent or the voltage applied to the strip 38.

In either the polarization rotator 26A or the polarization coupler 24A,other stress-inducing structures can be used instead of the strip 38.For example, the top surface 40 of the substrate 10 can have wallsforming a trench 50 transversely displaced from the first waveguide 22Aas shown in FIG. 12, or walls forming a ledge 52 as shown in FIG. 13.The ledge 52 or trench 50 can be filled with a material, which may ormay not have other structures embedded within it. In all these cases,the essential feature is that the principal axis of a birefringentportion 27A of the first waveguide 22A be rotated relative to thepolarization vector of light provided by the laser 20A feeding thatwaveguide 22A. This is achieved by passing the waveguide 22A through aregion of the substrate 10 in which the local stress field has rotatedthe principal axis of the material relative to this polarization vector.

A ledge 52 or trench 50 in the substrate 10 can be formed by depositinga mask to cover those areas of the substrate 10 that are not to beetched. The substrate 10 is then placed under tension or compression soas to cause a uniform stress field within the substrate 10. Thesubstrate 10 is then etched, using known dry etching techniques, such asreactive ion etching or etching with an inductively coupled plasma, orusing known wet etching techniques such as etching with HF. The presenceof the ledge 52 or trench 50 creates a local perturbation the stressfield, which causes the principal axis within the birefringent portion27A to rotate. When a wave travels through a portion of a waveguide 22Athat extends through the perturbed stress field, the polarization ofthat wave is rotated.

An initial uniform stress field within the substrate 10 can also beformed during fabrication of the waveguide 22A by heating the substrate10, depositing a material thereon, and then cooling the substrate 10 andthe deposited material. The deposited material can also be within theinterior of the substrate 10 or it can be a film, such as a dielectricfilm, deposited on the surface of the substrate 10. To the extent thatthe substrate 10 and the deposited material have different coefficientsof thermal expansion, there will be a stress field within substrate 10.Examples of materials include those that have a high coefficient ofthermal expansion, for example metals, glass compositions having a highcoefficient of thermal expansion, such as boron doped silica, orpolymers having a high coefficient of thermal expansion. The formationof a trench 50 or ledge 52 will then locally perturb this stress fieldand thereby locally rotate the principal axis of the material throughwhich the waveguide 22A is to pass.

A laser diode typically emits linearly polarized light. In someembodiments, the beams provided by the laser diodes 20A-H enter theirrespective waveguides 22A-H with their respective polarization vectorsoriented in the same direction. In these embodiments, polarizationrotators are used to rotate the polarization vectors of one laser diode20A, C, E, G of each of the laser diode pairs 20A-B, 20C-D, 20E-F,20G-H.

In other embodiments, no polarization rotator is necessary because thebeams enter the substrate 10 with the desired polarizations. Forexample, if one laser diode 20A, C, E, G of each of the laser diodepairs 20A-B, 20C-D, 20E-F, 20G-H may be physically rotated ninetydegrees relative to that of the other laser 20B, D, F, H in that pair.Or, a birefringent film can be placed in the path of the beam emitted byone laser 20B, D, F, H in each pair, for example where that beam entersthe input coupling region 14. In either case, the polarizations of pairsof beams entering the input coupling region 14 are rotated relative toeach other outside the substrate 10. In both these cases, the substrate10 need not include any polarization rotators 26A.

One application of the optical multiplexers 8A-D as described herein isto provide a broadband polarization-independent pump beam for Ramanamplification. In Raman amplification of a signal beam, a high intensitypump beam is made to propagate with the signal beam through a waveguide.The difference between the wavelengths of the pump beam and the signalbeam is chosen such that energy is transferred from the pump beam to thesignal beam, thus amplifying the signal beam.

A difficulty associated with Raman amplification is that the extent towhich energy is transferred between the pump beam and the signal beamdepends in part on the difference between their polarization states.Because the polarization state of the signal beam is unpredictable, theextent to which the signal beam is amplified is also unpredictable.

In the illustrated optical multiplexers 8A-D, the output waveguide 30carries four pump beams, each at a different wavelength. Each pump beamhas both a principally-polarized component and an orthogonally-polarizedcomponent. The resulting output beam of the optical multiplexers 8A-D isthus depolarized. As a result, even if the signal beam polarization isunknown, there will always be a component of the pump beam having apolarization state oriented to transfer some energy to the signal beam.

In some cases, a discontinuity may form in the optical system with whichthe optical multiplexer 8A-D is used. Such a discontinuity typicallyreflects light back toward the optical multiplexer 8A-D. It is thereforeuseful to detect such a reflection so that the laser diodes 20A-H can beshut down. To enable such detection, a multiplexer 8A can include anoptional output tap 54 in optical communication with the first waveguide22A. The output tap 54 is connected to a back-reflection detector (notshown) that is configured to immediately shut down the laser diodes20A-H upon detection of a reflection.

The optical multiplexer 8A-D thus integrates polarization coupling androtation into a single substrate 10. Optionally, the laser diodes 22A-Hcan themselves be grown on the substrate 10, thereby eliminating theneed to provide for external optical coupling to the laser diodes 22A-H.

An optical multiplexer 8A-D as described herein has many applicationsother than those described above. For example, the multiplexer inaddition to providing a depolarized multi-wavelength pump beam to aRaman amplifier, the optical multiplexer 8A-D can be a pump multiplexerof single or multiple wavelengths for erbium-doped fiber amplifiers. Theoptical multiplexer 8A-D can also be used to multiplex light havingvarious polarizations in optical transportation systems, in test andmeasurement equipment, and in illumination and imaging systems.

Other embodiments are within the scope of the appended claims.

1. A polarization coupler comprising: a substrate; a stress-inducingfeature disposed to generate a stress field in the substrate; a firstwaveguide having a first-waveguide coupling portion that passes throughthe stress field; a second waveguide having a second-waveguide couplingportion that passes through the stress field; a first-waveguideperiodic-structure in optical communication with the first-waveguidecoupling portion; and a second-waveguide periodic-structure in opticalcommunication with the second-waveguide coupling portion.
 2. Thepolarization coupler of claim 1, wherein the first-waveguideperiodic-structure comprises a grating.
 3. The polarization coupler ofclaim 2, wherein the grating comprises a long-period grating.
 4. Thepolarization coupler of claim 2, wherein the grating comprises ashort-period grating.
 5. The polarization coupler of claim 1, whereinthe first-waveguide periodic-structure and the second-waveguideperiodic-structure are aligned with each other.
 6. The polarizationcoupler of claim 1, wherein the first and second periodic-structures areoffset from one another.
 7. The polarization coupler of claim 1, whereinthe first-waveguide periodic-structure and the second-waveguideperiodic-structure are disposed to be in optical communication with eachother.
 8. The polarization coupler of claim 1, wherein thestress-inducing feature comprises a stress-inducing strip disposed on asurface of the substrate.
 9. The polarization coupler of claim 8,wherein the stress-inducing strip is disposed above the first waveguide.10. The polarization coupler of claim 1, wherein the stress-inducingfeature is disposed above the first waveguide.
 11. The polarizationcoupler of claim 8, wherein the stress-inducing strip extends across thefirst and second waveguides.
 12. The polarization coupler of claim 8,wherein the stress-inducing strip is selected to have a thermalexpansion coefficient that differs from a thermal expansion coefficientof the substrate.
 13. The polarization coupler of claim 1, wherein thefirst waveguide has a first-waveguide cross-section, and the secondwaveguide has a second-waveguide cross-section that differs from thefirst-waveguide cross-section.
 14. The polarization coupler of claim 1,wherein the first waveguide has a first-waveguide cross-section, and thesecond waveguide has a second-waveguide cross-section that has the samegeometry as the first-waveguide cross-section.
 15. The polarizationcoupler of claim 1, wherein the stress-inducing feature comprises apiezoelectric strip bonded to the substrate, the piezoelectric stripbeing configured to deform in response to an applied voltage, and tothereby selectively apply stress to the substrate.
 16. The polarizationcoupler of claim 1, wherein the stress-inducing feature comprises aresistive strip bonded to the substrate, the resistive strip beingconfigured to deform in response to heat generated by an appliedcurrent, and to thereby selectively apply stress to the substrate.
 17. Amethod for combining a first light-wave having a first polarization anda second light-wave having a second polarization, the method comprising:inducing a stress field in a substrate; guiding the first light-wavethrough the stress field in a first waveguide, the first waveguidehaving a first-waveguide coupling-portion in optical communication witha first-waveguide periodic-structure; guiding the second light-wavethrough the stress field in a second waveguide, the second waveguidehaving a second-waveguide coupling-portion in optical communication witha second-waveguide periodic-structure.
 18. The method of claim 17,wherein inducing a stress-field comprises providing a stress-inducingstrip on the substrate.
 19. The method of claim 18, wherein inducing astress-field further comprises forming the stress-inducing strip from amaterial having a thermal expansion coefficient that differs from athermal expansion coefficient of the substrate.
 20. The method of claim17, further comprising providing a second waveguide having asecond-waveguide cross-section that differs from a first-waveguidecross-section of the first waveguide.
 21. The method of claim 17,further comprising providing a second waveguide having asecond-waveguide cross-section that has the same geometry as afirst-waveguide cross-section of the first waveguide.
 22. The method ofclaim 18, wherein providing a stress-inducing strip comprises disposingthe stress-inducing strip on the substrate above one of thefirst-waveguide coupling-portion and the second waveguide-couplingportion.
 23. The method of claim 17, wherein inducing a stress field inthe substrate comprises providing a piezoelectric strip bonded to thesubstrate, the piezoelectric strip being configured to deform inresponse to an applied voltage, and to thereby selectively apply stressto the substrate.
 24. The method of claim 17, wherein inducing a stressfield in the substrate comprises providing a resistive strip bonded tothe substrate, the resistive strip being configured to deform inresponse to heat generated by an applied current, and to therebyselectively apply stress to the substrate.
 25. An integrated opticalcircuit comprising: a substrate having a birefringent portion; a firstwaveguide having a first-waveguide coupling-portion that passes throughthe birefringent portion; a first-waveguide periodic-structure inoptical communication with the first-waveguide coupling-portion; asecond waveguide having a second-waveguide coupling-portion that passesthrough the birefringent portion; and a second-waveguideperiodic-structure in optical communication with the second-waveguidecoupling-portion.